The present invention relates generally to systems and methods for characterizing defects on a sample.
Particular types of structures are difficult to analyze. Typically, a plurality of contacts or vias are imaged with a scanning electron microscopy (SEM) inspection tool. An electron beam is scanned across a sample. Secondary and backscattered electrons are emitted from the scanned sample in response to the electron beam. The emitted electrons are detected from the scanned sample to generate an image of the surface, including the contacts or vias. If the contacts or vias are coupled to the same underlying substrate or conductive metal layer, the contacts or vias are expected to have a same intensity of emitted electrons when there are no defects present. Defective contacts or vias are expected to emit a different intensity of electrons than the non-defective vias. As a result of these different intensities, defective contacts or vias have a different intensity or brightness than non-defective contacts or vias within the image generated on an SEM tool. Accordingly, defective contacts or vias may be defined as contacts or vias that have a significantly different intensity than a majority of the scanned contacts or vias or as compared to a known defect-free scanned contacts or via.
There are a number of possible xe2x80x9croot causesxe2x80x9d for a defective contact or via. For example, two different defect types may have a same appearance during an SEM inspection. Additionally, since contacts or vias have relatively high aspect ratio, they are difficult to image with an SEM system and may result in non-killer or xe2x80x9cfalsexe2x80x9d defects during a voltage contrast inspection. In one example, the captured defects may include xe2x80x9ckiller defectsxe2x80x9d which are likely to cause a device to fail, as well as non-killer or xe2x80x9cfalsexe2x80x9d defects which are not likely to cause a device to fail.
It is also desirable to determine the root cause of a potentially defective contact or via. For example, it is desirable to determine whether the potentially defective contact or via found during the voltage contrast inspection contains a plug defect or is a xe2x80x9cfalsexe2x80x9d defect. Currently, this determination is performed by inspecting a cross section of a potentially defective via for a plug defect. A cross section of a defective via is typically obtained using a focused ion beam cross section. The cross section of the contact or via is then imaged with a scanning electron microscopy (SEM) to locate the cause of the defect. In sum, the root cause of a defective contact or via is determined by a focused ion beam cross sectioning and imaging of the potentially defective contact or via. Although this technique succeeds in determining a root cause for a potentially defective via, the sample is destroyed during such technique. This procedure may waste a wafer sample, even though there are only xe2x80x9cfalsexe2x80x9d defects present. Additionally, the focused ion beam cross sectioning is time consuming.
Accordingly, there is a need for improved apparatus and methods for efficiently detecting and analyzing potential defects associated with a contact or via and the like.
In general terms, the present invention provides apparatus and methods for characterizing a potential defect by analyzing the X-ray count(s) of one or more material components (such as the ratio of oxygen over silicon) as emitted from a structure under test in response to electrically charged particles, such as an electron beam scan. For example, it may be determined whether a potentially defective contact or via has a SiO2 plug defect by comparing an X-ray count ratio of oxygen over silicon of the defective via with an X-ray count ratio of a known defect-free reference via. If the defective contact or via has a relatively high ratio (more oxygen than silicon) as compared to the reference contact or via, then it may be determined that a SiO2 plug defect is present within the defective contact or via. Otherwise, the contact or via may be defined as having a different type of defect (e.g., not a SiO2 plug defect) or defined as resulting in a xe2x80x9cfalsexe2x80x9d defect. Accordingly, specific embodiments of the present invention may be utilized to filter xe2x80x9cfalsexe2x80x9d defects from a defect sample. The X-ray counts of other types of materials may also be analyzed to determine root cause of potential defects, and material selection depends on the particular type of defect being analyzed and structure under test. In other embodiments, the size of a defect may also be determined by analyzing the X-ray counts of one or more materials produced in response to an electron beam scan over a structure under test.
In one embodiment, a method of characterizing a potential defect of a semiconductor structure is disclosed. A charged particle beam is scanned over a structure which has a potential defect. X-rays are detected from the scanned structure. The X-rays are in response to the charged particle beam being scanned over the structure. The potential defect of the scanned structure is characterized based on the detected X-rays.
In a specific implementation, the characterizing operation is based on a ratio of a first X-ray intensity for a first material over a second X-ray intensity for a second material, and the first and second X-ray intensities are obtained from the detected X-rays from the scanned structure. In a further implementation, the scanned structure is a first via or a first contact.
In a further aspect, a charged particle beam is scanned over a reference via. X-rays are detected from the scanned reference via. The X-rays are in response to the charged particle beam being scanned over the reference via. The potential defect is characterized by comparing the first ratio from the scanned first via or contact to a second ratio from the scanned reference via. The second ratio is a third X-ray intensity for the first material over a fourth X-ray intensity for the second material, and the third and fourth X-ray intensities are obtained from the detected X-rays from the scanned reference via
In one aspect, the potential defect is located based on the ratio of the first X-ray intensity for the first material over the second X-ray intensity for the second material. In a further aspect, a charged particle beam is scanned over a plurality of second vias or contacts. X-rays are detected from the scanned second vias or contacts, and the X-rays are in response to the charged particle beam being scanned over the second vias or contacts. Characterizing the potential defect of the first via or contact is accomplished by determining whether the first ratio from the scanned first via or contact significantly differs from a majority of second ratios calculated for the second vias. The second ratios of the plurality of vias are each calculated by dividing a third X-ray intensity for the first material by a fourth X-ray intensity for the second material, and the third and fourth X-ray intensities are obtained from the detected X-rays from each of the scanned second vias. In an alternative implementation, the potential defect is located by a voltage contrast inspection of a plurality of vias.
In one implementation, the first and second X-ray intensity are X-ray count values. In a preferred embodiment, the charged particle beam has a spot diameter substantially equal to a diameter of the via. In another aspect, characterizing the potential defect includes determining whether the scanned first via or first contact contains a plug defect, e.g., caused by an under-etching of the via or contact. (A xe2x80x9cplug defectxe2x80x9d is defined herein as any type of unwanted material which is present within the via or contact). In a further aspect, characterizing the potential defect further includes determining that the potential defect is a real defect when it is determined that the scanned first via or contact contains a plug defect and determining that the potential defect is a false defect when it is determined that the scanned first via or contact does not contain a plug defect. In an alternative further aspect, when it is determined that the first via or contact contains a plug defect, a size of the plug defect is determined based on the ratio of the first X-ray intensity for the first material over the second X-ray intensity for the second material.
In a specific implementation, characterizing the potential defect is accomplished by determining that the potential defect is a plug defect when the ratio is above a predetermined threshold. In an alternative implementation, characterizing the potential defect is accomplished by determining that the potential defect is a plug defect when the ratio is below a predetermined threshold.
In another embodiment, the invention pertains to an apparatus for characterizing a potential defect of a semiconductor structure. The apparatus includes a beam generator operable to direct a charged particle beam towards a structure and a detector positioned to detect X-rays from the structure in response to the charged particle beam. The apparatus further includes a processor arranged to perform one or more of the above described method operations.
These and other features of the present invention will be presented in more detail in the following specification of the invention and the accompanying figures, which illustrate by way of example the principles of the invention.